In recent years, omission of coating after injection molding of a colored resin is becoming popular for production of resinous automotive parts from viewpoints of cost reduction and/or environmental consideration. Particularly, as omission of coating has become possible due to improvement in mold technology etc. also for silver-metallic color parts for which omission of coating was difficult because of technical difficulty such as the occurrence of a weld line, application to versatile product parts is expanded. The omission of coating is desirable, for example, for interior resin parts of automobiles or four-wheel cars, such as an instrument panel garnish, a door trim garnish, and a shift garnish, for the purpose of cost cutting or reduction of a volatile organic compound (VOC). As a coating-free resin material, AES resin (acrylonitrile-ethylene-styrene resin, or acrylonitrile/ethylene-propylene-diene/styrene resin) has preferable characteristics from the viewpoints of colorability, scratch resistance, light resistance, etc. (Patent document 1). However, the AES resin has low flowability, and is therefore unsuitable for shaping of large-sized parts or elongated parts, and also its application to parts has been restricted since it has inferior impact resistance or chemical resistance.
Therefore, coating-free parts of polypropylene-based resin (hereafter, also referred to as “PP(-based) resin”) which is excellent in flowability (melt-moldability), impact resistance, and chemical resistance had been desired, but it is inferior in colorability and scratch resistance because of a low surface hardness, so that commercial production thereof has been restricted. More specifically, the conventional material designing of PP-based resin in automotive application has been principally made of a ternary system based on ICP (impact polymer) formed of ethylene-propylene copolymer rubber (hereafter, sometimes also referred to as “EPR”) and blended with an elastomer and an inorganic filler such as talc so as to provide a compatibility between impact resistance and rigidity. However, ICP becomes opaque due to inclusion of an EPR component, and becomes further cloudy by adding an inorganic filler, such as talc. Coloring by inclusion of a colorant or a luminous material is advantageously achieved if the base resin has a better transparency, the material designing based on the ternary system provides only white-blurred color which is poor in aesthetic appearance, so that it has not been suitable for providing a high luminosity color, such as silver-metallic color. Further, although the scratch resistance is advantageously attained at a higher surface hardness, the material designing based on the ternary system is caused to have a lower hardness due to the EPR component, and is liable to be scratched or marred easily. Furthermore, when talc is added, the scratched part is further conspicuously whitened. For these reasons, the conventional PP-based resin has been restrictively adopted for parts which are seldom touched by human hands after applying crimp processing for improving scratch resistance. For the parts requiring higher scratch resistances and aesthetic appearance, such as an instrument panel garnish and a shift garnish, such a measure as by crimping alone is insufficient. Therefore, it is a current practice that AES resin having a high surface hardness and excellent colorability has been more frequently adopted as a coating-free resin although it is problematic in flowability and formability.